The present invention relates to a novel expandable vinyl chloride resin composition for foaming under atmospheric pressure. More particularly, it relates to a vinyl chloride resin composition capable of giving a foam of a high expansion ratio and having an excellent rubber-like elasticity by forming under atmospheric pressure.
Usually vinyl cloride resin foams have been prepared by an atmospheric pressure foaming process or a pressure foaming process.
The atmospheric pressure foaming process usually consists of the steps of forming an expandable vinyl chloride resin composition into a preform such as sheet at a temperature of less than the decomposition temperature of the foaming agent employed and heating the preform under an atmospheric pressure at a temperature of not less than the decomposition temperature of the foaming agent to expand the preform by means of the gas generated by the decomposition of the foaming agent. The pressure foaming process usually consists of the first foaming step wherein an expandable vinyl chloride resin composition is heated and melted under an elevated pressure at a temperature of not less than the decomposition temperature of the foaming agent so that the gas generated by the decomposition of the foaming agent is dispersed finely into the molten composition, and the second foaming step wherein the foamed composition is finally expanded under an atmospheric pressure.
According to the atmospheric pressure foaming process, vinyl chloride resin foams can be produced by continuous foaming employing a calender or an extruder. However, the process involves such a technical difficulty that when a foam of a high expansion ratio, i.e. a low specific gravity is tried to be obtained from an expandable composition employing a usual vinyl chloride homopolymer or copolymer, the cells of the foam become coarse ununiformly and disintegrated, resulting in failure in obtaining a desirable foam. Therefore, the fact is that only foams of lower expansion ratios of upmost 5 to 6 can be obtained. Moreover, a thicker foam (for instance, 10 mm. in thickness) obtained by the atmospheric pressure foaming process has a drawback that when the foam is subjected to compression, a strain remains in the foam and the cushiony property of the foam is not comparable with that of polyurethane foam or polyolefin foam.
On the other hand, according to the pressure foaming process, vinyl chloride resin foams of high expansion ratios, for instance, having a specific gravity of 0.03 to 0.04 g./cm..sup.3 can be readily obtained. However, the process is poor in productivity since the foaming in the process is carried out by batch process. Therefore, if possible, it is desirable to produce vinyl chloride resin foams of high expansion ratios by the atmospheric pressure foaming process.
For the purpose of settling the above-mentioned problems of the atmospheric pressure foaming process, there has been proposed an expandable vinyl chloride resin composition composed of a vinyl chloride copolymer having active hydrogens such as a copolymer having hydroxyl groups or carboxyl groups, as a main resin ingredient and a crosslinking agent, a foaming agent and a plasticizer. However, the composition has a drawback that the foaming of the composition requires a precise foaming condition. That is, the reaction of the vinyl chloride copolymer having the active hydrogens, i.e. the hydrogen of the hydroxyl or carboxyl group with the crosslinking agent proceeds rapidly at a temperature in the vicinity of the decomposition temperature of the foaming agent, whereby the melt viscosity of the composition during the formation of cells is influenced significantly. As a result, if the foaming takes place ahead the crosslinking, the resulting cells become coarse, while if the crosslinking takes place ahead the foaming, the foaming is hindered. Therefore, it is very difficult to obtain a highly expanded foam having uniform, fine cells from the above-mentioned composition.